1) Field of the Invention
The present invention relates to an interference canceling device used in a receiver for mobile communication or the like.
2) Description of Related Art
In mobile communications, both FDD (Frequency Division Duplex) systems and CDMA (Code Division Multiple Access) systems are employed. In the FDD system, it is possible to prevent the effects of interference by selecting frequency channels so as to exclude frequencies in which interference exits. In the CDMA system, however, because this system uses a broad frequency band for signal transmission, there is no means effective other than providing an interference canceling device within a receiver to directly eliminate interference.
FIG. 10 illustrates an example of an interference canceling device, which comprises a canceling circuit 10 provided in a main line 12. The canceling circuit 10 consists of M notch filters 14-i (M: a natural number larger than 2; and i: 1, 2, . . . M) connected in cascade.
Each notch filter 14-i at the i-th stage comprises a local oscillator 16 oscillating at a frequency which differs from the oscillation frequencies of local oscillators in other stages, a mixer (MIX) 18 for mixing a signal on the main line 12 (hereinafter referred to as xe2x80x9cmain signalxe2x80x9d) with the oscillation output from the local oscillator 16 to convert it to a signal having a lower intermediate frequency (IF), an IF notch filter 20 for filtering the IF output from the MIX 18, and a mixer MIX 22 for mixing the filtered output from the IF notch filter 20 with the oscillation output from the local oscillator 16 to revert it to a signal having the original radio frequency (RF).
The IF notch filter 20 in the notch filter 14-i is configured to eliminate specific frequency components from a main signal which has been converted into an IF. The frequency to be eliminated is fixed to an identical value for all the IF notch filters at every stage.
Further, the frequency band in which interference is considered to exist has a certain width. Therefore, in order to cover this band, the frequency at which the local oscillator of each stage oscillates is set to differ from one stage to another by an appropriate amount. It is possible to eliminate interference with regard to a frequency band having a certain width, by thus setting the oscillating frequency value of each oscillator in accordance with frequencies considered to include interference, and also with appropriate differences.
However, the device shown in FIG. 10 has certain drawbacks. Among these, the canceling circuit 10 is provided on the main line 12. Accordingly, when insertion loss and delay occurring in the canceling circuit 10 is large, a noise figure NF of the main signal output deteriorates and also adaptability over the whole system is lost. This problem is especially notable when the number of stages M of the notch filters is large. Also, in the device shown in FIG. 10, down conversion from RF to IF and up conversion from IF to RF are executed so as to facilitate signal processing. At the time of these conversions, noise generated at the local oscillator 16 is added to the main signal, and also distortion introduced at MIXs 18 and 22 is influential in deteriorating the channel quality.
To solve the above problems, it is appropriate to displace a canceling circuit 10A from the main line 12, as shown in FIG. 11. In the canceling circuit 10A, a signal which has been branched from a main signal by means of a directional coupler 24 is converted into a signal having an IF by the local oscillator 16 and the MIX 18, and is then filtered by an IF narrow band BPF 20A to extract frequency components in which interference is considered to exist. The filtered output from the IF narrow band BPF 20A is further converted into a signal having an RF by the MIX 22. Of the outputs from the MIX 22, sum frequency components, namely frequency components corresponding to those of a main signal are recombined, as a cancellation signal, with the main signal by a directional coupler 28. Further, at the time of this recombining, a control circuit 30 uses a directional coupler 32 to detect amplitude and phase of the main signal or detect whether or not there are signals at specific frequencies. On the basis of the detection result, phase shift and gain in a variable phase shifter 34 and a variable gain amplifier 36, respectively, are controlled such that at the specific frequencies, namely at frequencies including interference, the cancellation signal has an amplitude which is of the same level as that of the main signal and a phase which is opposite to that of the main signal. In this device, the aforementioned problems, such as quality deterioration of a main signal found in the device of FIG. 10, does not occur.
However, in the device shown in FIG. 11 a drawback remains in that a frequency band for interference elimination is much narrower than that in the device of FIG. 10. Specifically, the IF narrow band BPF 20A usually represents band pass characteristics in the vicinity of its pass band center frequency f0, as depicted in FIGS. 12A to 12C. In particular, phase characteristics have a certain gradient, as shown in FIG. 12C. Therefore, even if the aforementioned automatic control is executed in the circuit of FIG. 11 by the variable phase shifter 34 and the variable gain amplifier 36, phases of the cancellation signal and of the interference match only at one point in frequency (e.g., at f0) as shown in FIG. 13, and this leads to deterioration of cancellation characteristics due to phase rotation caused by frequency separation from the frequency f0. It is therefore difficult, or even impossible, to eliminate interference spreading over a frequency band having a certain width, for example, a frequency modulated wave, with such xe2x80x9cpin-pointxe2x80x9d cancellation characteristics.
It is therefore an object of the present invention to overcome disadvantageous pin-point cancellation characteristics in the prior art, thereby allowing sufficient cancellation of interference spreading in a frequency band having a certain width. In the present invention, this object is achieved by improving the configuration of a narrow band BPF constituting a canceling circuit.
In the present invention, the narrow band BPF in the canceling circuit is configured such that it represents substantially flat phase characteristics in a specific frequency band having a certain width. A signal which has been branched from a main signal is filtered by this narrow band BPF to generate a cancellation signal in such a manner that a relation of xe2x80x9csubstantially the same amplitude level and opposite phasesxe2x80x9d can be established between the main signal and the cancellation signal over the entire range of the above-mentioned specific frequency band. Accordingly, within the above-mentioned specific frequency band, phase rotation of a cancellation signal caused by separation from the specific frequency can be eliminated (or can be suppressed to a level which can be substantially disregarded), thereby solving a problem in the prior art resulting from such phase rotation. In other words, the present invention enables elimination of interference ranging over a frequency band having a certain width, for example, a frequency modulated wave.
A narrow band BPF having flat phase characteristics, which is one feature of the present invention, can be implemented as a filter comprising, for example, a plurality of signal paths having filtering characteristics different from one another; distribution means for distributing signals which have been branched from a main signal to the signal paths; and combining means for combining the outputs from the signal paths to generate a filtered output. For example, a narrow band BPF having a substantially flat phase characteristics can be obtained relatively easily by providing a main signal path having predetermined filtering characteristics and a delay signal path having a greater delay than that of the main signal path, signal attenuation of the delay path being set in accordance with the delay ratio of the two paths. The main signal path and the delay signal path may pass through separate circuits (namely, parallel paths) over the whole configuration, or may share a part (a filter of the like) of the configuration.
Further, it may be preferable to provide a plurality of canceling circuits having the above-mentioned specific frequency bands set to differ from one another, such that they are parallel to one another, when seen from the main line via main line signal branching means and main line signal recombining means such as directional couplers or the like. This configuration enables interference elimination over a plurality of frequency bands. In particular, because the present invention expands the range of interference elimination from a pin-point frequency as in the conventional art to a specific frequency band having a certain width, provision of such a plurality of canceling circuits has a great significance.
It may also be preferable to provide a plurality of canceling circuits having substantially the same specific frequency band or having specific frequency bands different from one another, such that they are connected in series with one another as seen from the main line via main line signal branching means and main line signal recombining means such as directional couplers or the like. This configuration allows operation such as for selectively activating the plurality of canceling circuits. Namely, it is possible to activate only one canceling circuit when interference is weak while activating all the circuits when interference is strong. Thus, interference cancellation can be easily executed over a range from weak interference to strong interference. It is to be noted that a canceling circuit at each stage may comprise a single circuit or a group of canceling circuits connected as described in the previous paragraph.